Both the United States and the Russian Federation have established orbiting satellite navigation systems, and the Europeans are planning a system. The GPS system and the GLONASS system each employ a constellation of orbiting satellites which provide signals to receivers on the earth (ground, airborne, marine) and in space which are used to determine precise three-dimensional positions and time (e.g., latitude, longitude, altitude, time). Such signals can be used, for example, for navigation, surveying, timing, positioning and for measuring dimensional and time changes. Both the GPS and GLONASS systems each use two separated bands of frequencies in the L-band portion of the electromagnetic spectrum which have been allocated for radionavigation satellite services by the International Telecommunication Union (ITU).
In the case of both the GPS system and the GLONASS system, the frequency bands are designated L1 for the higher frequency band and L2 for the lower frequency band. A detailed description of the signal structure used for the GPS system is provided in Kayton, M. and W. R. Fried, Avionics Navigation Systems, 2d Ed., Chapter V, Satellite Radionavigation by A. J. Van Dierendonck, Section 5.5.5 GPS Signal Structure, pp. 213-282, John Wiley and Sons, Inc., New York, N.Y., 1997, which description is hereby incorporated by reference herein.
Referring to the drawings, FIG. 1 shows the existing GPS signal structure, generally designated by reference numeral 10. In FIG. 1, C/A designates the existing GPS coarse/acquisition code modulation on the L1 carrier, while P/Y indicates the GPS precise/encrypted code modulation of the L1 and L2 carriers, and L2.phi. indicates the "carrier phase" part of the P/Y-code signal at L2 that is authorized for civil use (for ionospheric correction).
For the L1 band, the signal energy of the C/A-code is concentrated at the center of the bands 12, with very little C/A-code energy at or near the P/Y-code nulls 14,16. For the L2 band, there is no C/A-code signal centered in the band 18 and no C/A-coded signal at or near the P/Y-code nulls 20,22.
Throughout the drawings, the frequency occupancies of the bands (to their first spectral nulls) are shown, not the shape of the waveform, or signal power distribution, of each band. Those skilled in the art who have reviewed the present disclosure will readily appreciate the waveform shape in each situation.
Known systems have the following drawbacks. First, accuracy is normally to within several meters; accuracy to within centimeters or decimeters adds considerable cost and complexity and is reliably achieved only by the use of differential measurements of the carrier phases of the received signals. One problem in achieving high accuracy is the problem of resolving the integer wavelength ambiguity associated with the carrier phase measurements. Second, modulations provided for civilian and military uses have maxima near one another (or collocated in frequency), which is undesirable for some military purposes.